1. Field of the Invention
This invention pertains to handheld sound projection devices. More particularly, the present invention relates to a device and method for enhancing a bullhorn with directionally projected light in combination with a directional parametric speaker.
2. State of the Art
Outdoor sound projection and amplification is typically accomplished with a megaphone or bullhorn device capable of extending the distance of speech projection. Such focusing devices are necessary because the human voice quickly dissipates in an open environment. This arises in part from the fact that the human speech mechanism is extremely effective in omnidirectional sound projection. The complex resonant structure of the skull, mask of the face and vocal column are amazingly proficient in radiating sound in a generally omnidirection manner.
A megaphone operates to more effectively match the interface between an open environment and the mouth of the speaker. By channeling the sound through an expanding cone, the compression waves that must carry the sound are restricted in path and provided with an enlarging planar wave front diameter. By the time the wave front is enlarged to the opening size of the megaphone, a strong directional element is achieved, enabling a projection area of an enlarging wedge, rather than the conventional omnidirectional propagation pattern.
Despite the increased distance range of the megaphone, an unaided voice is quickly attenuated in proportion to the square of the distance. A bullhorn complements the megaphone structure with electronic voice amplification. By boosting the amplitude of the voice with a conventional amplifier circuit, a significantly extended range of hearing is achieved. Nevertheless, the pattern of propagation is still very divergent once the sound waves clear the horn structure. This results in a general broadcast to the surrounding area, without ability to limit the listening audience. The inconvenience of general dissemination of the amplified voice communication has become accepted as an inherent limitation of a bullhorn or similar sound projection system. For example, a police helicopter equipped with a PA system can broadcast emergency messages; however, they are broadcast generally rather than being directable to a specific target area. At night, such messages may alarm or even awaken persons who need not be involved. Other messages generally broadcast can create confusion where people listen who have no interest or knowledge of the matter communicated.
A more recent technology involving directional sound has developed as part of an attempt to reproduce sound without use of a moving diaphragm such as is applied in a conventional bullhorn. This second sound propagation approach includes technologies embodied in parametric speakers, acoustic heterodyning, beat frequency interference and other forms of modulation of multiple frequencies to generate a new frequency.
In theory, sound is developed by the interaction in air (as a nonlinear medium) of two ultrasonic frequencies whose difference in value falls within the audio range. Ideally, resulting compression waves would be projected within the air as a nonlinear medium, and would be heard as pure sound. An interesting property of parametric sound generation is enhanced directionality. Despite significant publications on ideal theory, however, general production of sound for practical applications has alluded the industry for over 100 years. Specifically, a basic parametric or heterodyne speaker has not been developed which can be applied in general applications in a manner such as conventional speaker systems.
A brief history of development of the theoretical parametric speaker array is provided in "Parametric Loudspeaker--Characteristics of Acoustic Field and Suitable Modulation of Carrier Ultrasound", Aoki, Kamadura and Kumamoto, Electronics and Communications in Japan, Part 3, Vol. 74, No.9 (March 1991). Although technical components and the theory of sound generation from a difference signal between two interferring ultrasonic frequencies is described, the practical realization of a commercial sound system was apparently unsuccessful. Note that this weakness in the prior art remains despite the assembly of a parametric speaker array consisting of as many as 1410 piezoelectric transducers yielding a speaker diameter of 42 cm. Virtually all prior research in the field of parametric sound has been based on the use of conventional ultrasonic transducers, typically of bimorph character.
U.S. Pat. No. 5,357,578 issued to Taniishi in October of 1994 introduced alternative solutions to the dilemma of developing a workable parametric speaker system. Hereagain, the proposed device comprises a transducer which radiates the dual ultrasonic frequencies to generate the desired audio difference signal. However, this time the dual-frequency, ultrasonic signal is propagated from a gel medium on the face of the transducer. This medium 20 "serves as a virtual acoustic source that produces the difference tone 23 whose frequency corresponds to the difference between frequencies f.sub.1 and f.sub.2." Col 4, lines 54-60. In other words, this 1994 reference abandons direct generation of the difference audio signal in air from the face of the transducer, and depends upon the nonlinearity of a gel medium to produce sound. This abrupt shift from transducer/air interface to proposed use of a gel medium reinforces the perception of apparent inoperativeness of prior art disclosures, at least for practical speaker applications.
Therefore, although the parametric speaker has created interest, it has seemingly been restricted to scientific curiousity. The development of practical applications and products has been very limited. The efficiency of such systems has apparently not been adequate to suggest its utility in applications in combination with a megaphone or bullhorn.